1. Technical Field
This invention relates to the application of genetic engineering techniques to plants. Specifically, the invention relates to compositions and methods for enhancing expression of proteins in plant plastids.
2. Background
The plastids of higher plants are an attractive target for genetic engineering. Plant plastids (chloroplasts, amyloplasts, elaioplasts, etioplasts, chromoplasts, etc.) are the major biosynthetic centers that, in addition to photosynthesis, are responsible for production of industrially important compounds such as amino acids, complex carbohydrates, fatty acids, and pigments. Plastids are derived from a common precursor known as a proplastid and thus the plastids present in a given plant species all have the same genetic content. Plant cells contain 500-10,000 copies of a small 120-160 kilobase circular genome, each molecule of which has a large (approximately 25 kb) inverted repeat. Thus, it is possible to engineer plant cells to contain up to 20,000 copies of a particular gene of interest which potentially can result in very high levels of foreign gene expression. In addition, plastids of most plants are maternally inherited. Consequently, unlike heterologous genes expressed in the nucleus, heterologous genes expressed in plastids are not pollen disseminated, therefore, a trait introduced into a plant plastid will not be transmitted to wild-type relatives.
There remains a need for improved regulatory elements for expression of genes in a plant plastid. To date, the expression signals used routinely for plastid transgene expression derive from endogenous plastid genes. The plastid expression signals are typically derived from promoter regions of highly expressed plastid genes such as the promoter regions from the 16S ribosomal RNA operon (Prrn), psbA gene (PpsbA) or the rbcL gene (PrbcL). The psbA and rbcL genes are highly transcribed, but their translation is controlled by tissue-specific and light-regulated factors which limits their usefulness. In the case of Prrn, a synthetic ribosome binding site (RBS) patterned after the plastid rbcL gene leader has been typically used to direct translation. However, this Prrn/RBS is translated inefficiently due to poor ribosome binding.
Plastids of higher plants present an attractive target for genetic engineering. As mentioned above, plastids of higher plants are maternally inherited. This offers an advantage for genetic engineering of plants for tolerance or resistance to natural or chemical conditions, such as herbicide tolerance, as these traits will not be transmitted to wild-type relatives. In addition, the high level of foreign gene expression is attractive for engineered traits such as the production of pharmaceutically important proteins.
Expression of nucleic acid sequences encoding for enzymes providing for herbicide tolerance as well as pharmaceutical proteins from plant plastid genome offers an attractive alternative to expression from the plant nuclear genome.
Relevant Literature
McBride et al. U.S. Pat. No. 5,576,198 and McBride et al. (1994) Proc Natl Acad Sci 91:7301-7305 reports the plastid expression system based on a two component system utilizing a nuclearly encoded T7 polymerase targeted to the plastid which activates a transgene controlled by the T7 bacteriophage gene 10 promoter. Svab et al. (1990) Proc Natl Acad Sci 87:8526-8530 reports the standard chloroplast transformation methods. Svab, et al. (1993) Proc Natl Acad Sci 90:913-917 reports the use of the aadA gene for use in selection of transplastomic plants on spectinomycin and streptomycin, as well as integration sequences. Zoubenko et al. (1994) Nuc Acid Res 22:3819-3824 reports the construct of vectors for use in plastid transformation.
Barry, et al., U.S. Pat. No. 5,627,061 describes the cloning of EPSPS nucleotide sequences from several sources including Agrobacterium strain CP4 and methods for producing glyphosate tolerant plants. Kishore and Shah, Ann. Rev. Biochem. (1988) 57:627-663 reports the modification of DNA sequences for the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (hereinafter referred to as EPSP synthase or EPSPS) for the enhancement of glyphosate tolerance. Stalker et al., U.S. Pat. No. 4,810,648 describes the cloning and use of a nucleic acid sequence encoding for the bromoxynil degrading gene, nitrilase.
The present invention provides nucleic acid sequences useful in enhancing expression of a wide variety of genes, both eukaryotic and prokaryotic, in plant plastids. Furthermore, plastid expression constructs are provided which are useful for genetic engineering of plant cells and which provide for enhanced expression of the EPSP synthase proteins or the hGH protein in plant cell plastids. The transformed plastids should be metabolically active plastids, and are preferably maintained at a high copy number in the plant tissue of interest, most preferably the chloroplasts found in green plant tissues, such as leaves or cotyledons.
The plastid expression constructs for use in this invention generally include a plastid promoter region capable of providing for enhanced expression of a DNA sequence, a DNA sequence encoding an EPSPS protein or human growth hormone (hGH), and a transcription termination region capable of terminating transcription in a plant plastid.
The plastid promoter region of the present invention is preferably linked to a ribosome binding site which provides for enhanced translation of mRNA transcripts in a plant plastid.
The plastid expression construct of this invention is preferably linked to a construct having a DNA sequence encoding a selectable marker which can be expressed in a plant plastid. Expression of the selectable marker allows the identification of plant cells comprising a plastid expressing the marker.
In a preferred embodiment, vectors for transfer of the construct into a plant cell include means for inserting the expression and selection constructs into the plastid genome. The vectors preferably comprise regions of homology to the target plastid genome which flank the constructs.
The constructs of the present invention preferably comprise a promoter sequence linked to a ribosome binding site capable of enhancing the translation of mRNA transcripts in the plant plastid. The ribosome binding site is preferably derived from the T7 bacteriophage gene 10 leader sequence.
Of particular interest in the present invention is the high level of expression of nucleic acid sequences in plant plastids. Of particular interest is the high level expression of nucleic acid sequences encoding for enzymes involved in herbicide tolerance and encoding for pharmaceutical proteins.
The constructs of the present invention preferably comprise a DNA sequence encoding 5-Enolpyruvylshikimate-3-phosphate synthase (U.S. Pat. No. 5,633,435, the entirety of which is incorporated herein by reference), nitrilase, phytoene desaturase, aprotinin or a DNA sequence encoding human growth hormone (U.S. Pat. No. 5,424,199, the entirety of which is incorporated herein by reference).
Plant cell plastids containing the constructs are also contemplated in the invention, as are plants, plant seeds, plant cells or progeny thereof containing plastids comprising the construct.
The present invention also includes methods for enhanced expression of DNA sequences in plant plastids.
The invention also includes a method for the enhanced expression of an enzyme encoding hGH in plastids of the plant cell.
The present invention further includes methods for obtaining a protein expressed from a plant cell, including a plastid, having a non-methionine N-terminus. In addition, plant cells and plastids which include non-methionine N-terminus proteins are contemplated.
Thus, the present invention relates to a chimeric gene containing a coding sequence of a pharmaceutical protein, a plant plastid expression vector containing a promoter operably linked to a T7 Bacteriophage Polymerase gene 10 ribosome binding site capable of enhanced expression in a plant plastid operably linked to a herbicide tolerance or pharmaceutical coding gene, a plant transformation vector having inserted therein a herbicide tolerance or pharmaceutical coding gene expressed from a plastid promoter linked to a T7 Bacteriophage Polymerase gene 10 ribosome binding site, plant cells transformed using such vectors and plants regenerated therefrom which exhibit a substantial degree of expression of nucleic acid sequences and proteins and methods for producing such plants and such plants.